1. Field of the Invention
The invention relates to thermoplastic resin compositions and more particularly relates to improved copolyestercarbonate resins and their preparation.
2. Brief Description of the Prior Art
Aromatic copolyestercarbonate resins are a well known class of synthetic polymeric resins, generally prepared by the reaction of a polyhydric phenol with a carbonate precursor in the presence of an ester precursor; see for example U.S. Pat. No. 3,169,121. Although such resins have been found to be thermoplastically moldable under a broad range of molding conditions, only select copolyestercarbonate resin compositions are useful for commercial molding operations. This is due to the requirements of processability for a thermoplastic resin related to their melt flow behavior.
Furthermore, even though a given copolyestercarbonate resin may have the physical properties required for successful thermal molding, the product articles may be deficient in certain other physical properties otherwise desired. For example, the molded articles may lack a desired degree of impact strength, particularly at low temperatures.
Those skilled in the art have found that polyestercarbonate resins of a particular class exhibit a lowered glass transition (Tg) temperature and improved low temperature impact strengths. The Tg of the resin is reduced by the presence of aliphatic diester blocks.
The rheological behavior and reduced glass transition temperatures of these resins have use in injection molding. An example of such an application is in molded computer and business equipment housings. These parts require materials with reduced viscosities at shear rates experienced during mold filling in the injection molding process.
Examples of prior art descriptions of the above-described polyestercarbonate resins having aliphatic diester blocks are found in the U.S. Pat. Nos. 4,238,596; 4,238,597; 4,280,683; 4,628,081; 4,677,183; and 4,983,706. These resins, as described, were all prepared by the well-known interfacial polymerization method, which has many advantages over direct polymerization in organic solvents such as described in the U.S. Pat. No. 3,030,331.
Although the reaction conditions of the preparative interfacial polymerization processes may vary, several of the preferred processes typically involve dissolving or dispersing a dihydric phenol and an ester precursor reactant in aqueous caustic, adding the resulting mixture to a suitable water immiscible solvent medium and contacting the reactants with a carbonate precursor, such as phosgene, in the presence of a suitable catalyst and under controlled pH conditions. For example, in the process described in U.S. Pat. No. 4,983,706, phosgene is introduced into an aqueous solution of the alkali metal salt of a dicarboxylic acid ester precursor and the diphenol in methylene chloride, in the presence of a phase-transfer catalyst as well as a molecular weight regulator, usually a monofunctional phenol.
Polyestercarbonate resins containing aliphatic diester blocks, prepared by the prior art interfacial polymerization techniques are limited in two respects. First, the incorporation of short chain diacid moieties (i.e.; less than about 9 carbon atoms) is limited due to their high solubility in the aqueous phase. Generally, even with high excess diacid reactant, there is incomplete incorporation of the diacid moiety into the product copolymer. Secondly, undesirable amounts of diaryl carbonate are generally formed as a by-product of the polymerization. This contaminant is relatively volatile and will "plate out" during thermal processing onto molds and processing equipment. The contaminant will volatilize and then condense on the surfaces of molds, processing equipment and the molded articles themselves.
The process of the present invention incorporates into the resin backbone aliphatic diester blocks from a wide variety of aliphatic dicarboxylic acids through a rapid reaction with oligomeric bischloroformates, to form very reactive mixed carboxylic carbonic anhydride intermediates which immediately decompose to form ester and/or anhydride linkages. Any simple anhydrides formed during this step are easily converted to esters during the following polymerization step.
A reduction in diaryl carbonate is also obtained through the addition of a chainstopper to the reaction mixture subsequent to polymerization. Thus, the advantages of this process include the reduction of diaryl carbonate contaminate in the final polymer and the ability to fully incorporate any dicarboxylic acid chain moieties into the polymer.
Melt processable copolyestercarbonates having relatively high glass transition temperatures (on the order of 180.degree. C. or more) are described in the U.S. Pat. No. 4,310,652 (DeBons et al., Jan. 12, 1982). The term "low glass transition temperature" or "reduced Tg" as used in the present invention means a Tg of less than 145.degree. C.
The copolyestercarbonate resins produced by the method of the invention are useful to fabricate sound and image reproduction articles such as compact discs ("CDs") and the like.